High temperature vat dyeing process using magnesium compounds in the dye bath and compositions therefor



United States Patent HIGH TEMPERATURE VAT DYEING PROCESS USING MAGNESIUM COMPOUNDS IN THE DYE BATH AND COMPOSITIONS THEREFOR Jerry M. Mecco, Somerville, N. J., assignor to American Cyanamid Company, New York, N. Y., a corporation of Maine N0 Drawing. Application June 18, 1956, Serial No. 591,807

17 Claims. (Cl. 8--34) This invention relates to improved reduction baths containing alkali metal hydrosulfite and over-reduction buffers and the compositions for producing them.

This application is a continuation-in-part of my prior application Serial No. 318,336, filed November 1, 1952, now abandoned.

In ordinary vat dyeing there is used a reduction bath to transform the vat dye into its leuco form. This reduction bath is strongly alkaline and for the reducing material generally uses a metal hydrosulfite, usually sodium or zinc hydrosulfite. Numerous vat dyes in the past have shown themselves sensitive to over-reduction in dyeing at elevated temperatures. This constituted a serious drawback in the case of the sensitive dyes as there is a great economic advantage in dyeing at high temperature. In my prior Patents Nos. 2,548,543 and 2,548,544, both dated April 10, 1951, and in my Patent No. 2,576,846, dated November 27, 1951, I have described processes for the high temperature dyeing of vat dyestuifs which are sensitive to over-reduction using certain over-reduction bufi'ers. In the two patents dated April 10, 1951, the inorganic nitrites and halogenates are described and claimed, and in my Patent No. 2,576,846, dated November 27, 1951, organic nitrogen compounds in which the nitrogen is attached by at least one co-valent bond to oxygen and by at least one co-valent bond to an element other than oxygen or nitrogen are described and claimed. These compounds stabilized the dyestuffs and prevented over-reduction. The same compounds were also found to improve the dyeing with certain pyranthrone vat dyestuffs which are not sensitive to overreduction. Processes using nitrites and halogenates in the dyeing of these pyranthrone dyes are described and claimed in my prior Patents Nos. 2,548,545 and 2,548,546, both issued April 10, 1951, while in my Patent No. 2,576,847, dated November 27, 1951, the use of the organic nitrogen compounds referred to above is described and claimed in the dyeing of the pyranthrone dyes.

The stabilizing agents or over-reduction butters described solve the problem presented by over-reduction of sensitive vat dyestuffs in high temperature dyeing and improve the dyeing of the pyranthrone dyestuffs referred to above. However, this greatly improved result was obtained at the expense of increased hydrosulfite consumption and while the consumption of hydrosulfite is not so great as to render the improved dye processes uneconomical, the cost of the increased hydrosulfite consumption is nevertheless suflicient so that reduction of the excessive consumption is economically desirable.

The mechanism which results in increased hydrosulfite consumption is not fully determined, but appears to be due at least in part to chemical reaction with the stabilizer 'or over-reduction bufier.

According to the present invention, I have found that if a substantial amount of magnesium ions is introduced into the bath, the excess consumption of hydrosulfite at high temperatures is very greatly reduced, and as a result, on standing the hydrosulfite concentration is much higher than if the bath contained no magnesium ions. It is not known how the presence of magnesium ions reduces excessive hydrosulfite consumption, and therefore the invention is not restricted to a particular theory of how these ions behave. On the other hand it is definitely known that it is not a question of alkalinity, for the same result is not obtained when ions of other metals of the second group of the periodic system replace magnesium. It is possible that a magnesium salt or complex of the hydrosulfite may be formed which is less reactive with the stabilizers or over-reduction buflers than the alkali metal salts. However, though this may be a factor it seems probable that it is not the only factor because the magnesium ions are effective in amounts very materially less than the stoichiometrical equivalents necessary for forming magnesium hydrosulfite. On the other hand, the magnesium ions must be present in more than catalytic amounts. There is no sudden change as the ratio of magnesium ions to sodium hydrosulfite are reduced, but there is a practical lower limit below which there is not enough eifect to be of practical significance. Thus, when the molar ratio is less than 0.001 there is normally no useful reduction of hydrosulfite consumption, and this may be taken as a lower limit of practical utility. Hydrosuliite consumption decreases markedly with a larger molar ratio of magnesium ions to hydrosulfite but when the ratio exceeds 0.5, efficiency drops and as a 1:1 molar ratio is approached, the process becomes practically worthless.

The operation of the present invention is not limited to vat dyeing baths in which the vat dye is actually incorporated into the bath. Similar savings: in hydrosulfite consumption are obtainable when the bath contains only the stabilizing agent or over-reduction bufier and the hydrosulfite as in the case of certain pigment dyeing processes.

The preparation of baths has been described above in terms of adding ingredients to the bath. In many cases this is preferable, but it is alsopossible to produce compositions containing the vat dyestufi and the magnesium compound as the active components. These vat dye compositions are otherwise conventional and are usually marketed in the standard powder types containing small amounts of dispersing agents and extenders. It should be noted that the magnesium compound, such as magnesium sulfate, is similar to conventional extenders, and in preparing a type powder it replaces a corresponding amount of inert extender. Such compositions contain sufiicient magnesium so that when a vat dye bath is prepared in the normal manner, the magnesium counteracts the hydrosulfite consumption. Thus, it is possible to market a composition which can be used without any measurement of other ingredients to produce the dye baths of the present invention.

The over-reduction buffers are substantial equivalents and the present invention is therefore applicable regardless of whether there is a single butler present or a mixture of two or more of them. The effectiveness of the magnesium compound appears to depend entirely on its ability to produce magnesium ions in a dye bath. The other portion of the molecule does not exert any substantial effect and may either be oxidizing or reducing in nature. Thus, for example, magnesium thiosulfate is just as efiective as magnesium compounds having anions which are not reducing.

It is also possible to use the magnesium compounds of the present invention with other stabilizers such as trialkanolamines or manganese compounds. These other stabilizers are not claimed in the present application but constitute the subject matter of my co-pending application, Serial No. 258,281, filed November 26, 1951, and

Patented Aug. 20, 1957 3 the co-pending application of Frederick Fordemwalt, Serial No. 258,280, filed November 26, 1951, respectively, both now abandoned. When the other stabilizers are usedtogether with the magnesium compounds, the molar ratio of magnesium ions to hydrosulfite may fall slightly below 0.001 so long as the molar ratio of all of the stabilizers is in excess of the'lower limit.

The invention will be described in greater detail in conjunction with the-following specific examples, the parts being by weight unless otherwise specified- In the examples the commercial bath volume per weight of cotton dyed is used unless otherwise specified. This bath weight is approximately 10 times the'weight of cotton treated. As far as the bath weight is concerned, the present invention does not differ from standard practice, the essential. features being the molar ratio of magnesium ion to by drosulfite. At the beginning of each example this ratio or ratios are given under the symbol Mg/hs, the figures being carried only to the third decimal point.

Example 1 (Mg/hs 0.035)

0.500 part of a commercial dyeing paste of the blue vat dye having Color Index No. 1113, 6 parts of sodium hydroxide, 1.5 parts of sodium nitrite, and 0.250 part of MgSO4-7HzO were added to a dye beaker along with sufiicient water to make up a total of 200 parts. The contents were heated to a temperature of 200 F. and 6 parts of sodium hydrosulfite were added. A test made with a standard indigo solution at the end of 50 minutes showed the bath still contained approximately 2.2 parts of sodium hydrosulfite.

A corresponding dye bath containing no magnesium salt contained less than 0.7 part of hydrosulfite after only 20 minutes.

Example 2 (Mg/hs 0.018)

In a test carried out exactly as described in Example 1, except only 0.125 part of MgSO4-7H2O were used, the bath showed the presence of approximately 1.9 parts of hydrosulfite at the end of 50 minutes.

Example 3 (Mg/11s 0.007)

In a test carried out exactly as described in Example 1, except that only 0.05 part of MgSO4'7H2O were added, the bath contained approximately 0.7 part of hydrosulfite at the end of 50 minutes as compared with 0.7 parts at the end of only 20 minutes in the absence of the magnesium salt.

Example 4 (Mg/hs 0.054)

A dye bath was prepared to contain approximately 4 oZ./gal. sodium hydroxide, 1 oz./ gal. sodium nitrite, /3 oz./ gal. of the blue vat dye having Color Index No. 1113 and 1.0 oz./gal. of magnesium lignin sulfonate (2.46% Mg by weight). This was heated to the boil and 4 oz./ gal. of sodium hydrosulfite added. The bath showed a good hydrosulfite test, approximately 1 oz./gal., at the end of 20 minutes. A similar bath without the magnesium lignin sulfonate compound showed the presence of less than 0.5 OZ./ gal. of hydrosulfite in 10 minutes.

Example 5 (Mg/hs 0.071)

A dye bath was prepared to contain 3 parts sodium hydroxide, 3 parts sodium nitrite, 0.250 part of MgSO4'7H2O and 1 part of a commercial dyeing paste of the vat dye described in Example 1 of U. S. Patent No. 1,663,475, in

4 sufficient water for total of 200 parts. The above mixture was heated to C. and 3 parts of sodium hydrosulfite added. At the end of five minutes, a skein of cotton yarn amounting to 20 parts was introduced into the bath and dyed. At the end of 30 minutes, a titration with an indigo solution showed the bath still contained approximately 0.9 part of sodium hydrosulfite.

In a corresponding dyeing to which no MgSO4-7H2O had been added, less than 0.3 parts of hydrosulfite remained after only five minutes at 95 C.

Example 6 (Mg/hs 0.018)

A 5% dyeing of the vat dye having Color Index No. 11.13 was prepared on cotton yarn from a bath containing 4 oz./ gal. of sodium hydroxide, 4 oz./ gal. sodium hydrosulfite, 1 oz./ gal. sodium nitrite, and oz./ gal. of

MgSOe 7H2O 4 oz./ gal. sodium hydrosulfite 4 oz./gal. sodium hydroxide 1 oz./ gal. sodium nitrite 0.1 oz./ gal. of the blue vat dye having C. I. No. 1113 4 oz./ gal. MgSO4-7H2O After heating the bath for 20 minutes at 205 F a test With a standard indigo solution showed a sodium hydrosulfite content of 3 oz./gal.

Repeating the above but omitting the magnesium salt, a test with a standard indigo solution showed no sodium hydrosulfite remaining after heating one minute at 205 F.

Example 8 (Mg/hs 0.008)

Example 7 was repeated except magnesium chloride was used in place of the MgSO4-7H2O. The results were generally similar to those of Example 7.

Example 9 (Mg/hs 0.071, 0.035, 0.018, 0.007)

The general procedure of Example was repeated varying the magnesium sulfate as follows: /3 oZ./gal., /6 oZ./gal., A oz./ gal. and oz./ gal. The dye content was 0.3 oz./ gal. After dyeing for 30 minutes at 205 F., a test with a standard indigo solution showed that each of the baths still had a sodium hydrosulfite content of approximately 2 oz./gal., indicating that only half of the sodium hydrosulfite had been decomposed.

Example 10 (Mg/hs 0.5)

The procedure of Example 7 was repeated except 1 oz./gal. magnesium nitrite was used in place of the sodium nitrite and magnesium sulfate. A test with a standard indigo solution after 30 minutes at 205 F. showed a hydrosulfite content of 2 oz./gal., indicating that only half of the sodium hydrosulfite had decomposed.

Example 11 (Mg/hs 0.036) A dye bath was prepared to contain 4 o z./gal. of

sodium hydroxide, 0.1 oz./gal. sodium. nitrite and 0.1 oz./gal. of the commercial dyeing paste of the orange vat dye having Color Index No. 1098. The bath was heated to approximately 200 F. and 4 oz./gal. sodium hydrosulfite were added. The hydrosulfite content was still 3.3 oz./gal. atthe end of 30 minutes. 1 oz./gal. of sodium nitrite was then added. In four minutes time thehydrosulfite content dropped to less than 0.4 oz./gal., whereas in a corresponding bath which contained in addition 0.20 oz./gal. of MgSO4-7H2O, the bath contained five times as much hydrosulfite (2 oz./ gal.) after 5 minutes, showing that the presence of the magnesium salt reduced the rate of hydrosulfite consumptionto less than one-half that which occurred when no magnesium was used.

Example 12 (Mg/hs 0.025)

A bath was prepared to contain 4 oz./ gal. sodium hydroxide, 4 oz./ gal. sodium. hydrosulfite and 1 oz./ gal. sodium nitrite. At a temperature of 200 F., the rate of hydrosulfite consumption in this bath was approximately 0.4 oz./gal./min. In a corresponding bath to which 0.12 oz./gal. of MgSO4-7HzO had been added, the rate of hydrosulfite consumption was less than one-third as great aswhen no magnesium salt was present.

Example 13 (Mg/hs 0.006)

A dye bath containing 4 oz./gal. sodium hydroxide, 1 oz./gal. sodium nitrite, 0.08 oz./ gal. of a commercial paste of the green vat dye having Color Index No. 1101, and "0.03 oz./gal., of hydrated magnesium sulfate was heated to approximately 200 F. and 4 oz./ gal. sodium hydrosulfite then added. The rate of hydrosulfite consumption was less than one-half as great as in a similar bath which contained no magnesium salt.

A piece of 80 x 80 bleached unmercerized cotton cloth introduced at the end of 30 minutes and dyed for minutes became heavily dyed whereas a similar piece of bleached unmerc erized cotton cloth in the bath to which no magnesium sulfate had been added remained undyed.

Example 14 (Mg/hs 0.011)

A dye bath containing 4 oz./gal. NaOH, 0.1 oz./ gal. of commercial dyeing paste of the blue vat dye having Color Index No. 1113, 1 oz./ gal. of hydroxylamine hydrochloride, and 0.05 oZ./gal. of MgSO47H2O was heated to a temperature of approximately 200 F and 4 oz./ gal. of sodium hydrosulfite then added. The bath was kept at the same temperature for thirty minutes and then a swatch of 80 x 80 bleached unmercerized cotton cloth was introduced into the bath and dyed for 10 minutes at approximately 200 F. A good blue dyeing resulted.

A corresponding dyeing prepared in the same manner, excepting that no MgSO4-7HzO was added to the bath, was very weak, dull and gray.

Example 15 (Mg/hs 0.004)

0.22 oif/g'aLZmin. I

Example 16 r (Mg/hs 0.004)

In a test made as described in Example 15, except that 0.04 oz./gal. of a commercial dyeing paste of the blue vat dye having Color Index No. 1113 was present. The rate of hydrosulfite consumption was approximately 0.11 oz./gal./min. in the bath containing the magnesium salt and nearly twice that amount, 0.19 oz./g=al./min., in the bath containing no magnesium.

Example 17 (Mg/hs 0.004)

A bath which contained 4 oz./ gal. of NaOH, 2 oz./ gal. of hydroxylammonium sulfate and 0.01 oz./gal. of MgSOtr-7H2O was heated to approximately 200 F. and 4 oz./gal. of sodium hydrosulfite added. The rate of hydrosulfite consumption was approximately 0.22 oz./ gal./ min. magnesium salt present the hydrosulfite consumption was approximately 50% greater or 0.33 oz./gal./min.

Example 18 (Mg/hs 0.004)

In a test carried out as described in Example 17, except that each bath contained 0.04 oz./gal. of a commercial dyeing paste of the blue vat dye having Color Index No. 1113, the hydrosulfite consumption was almost twice as rapid in the bath which contained no magnesium as it was in the bath to which MgSO4-7H2O had been added.

Example 19 (Mg/hs 0.007)

Two dye baths were set up, each containing 4 oz./gal. of NaOH, 1 oz./gal. of NaNOz, 0.1 oz./gal. of a commercial dyeing paste of the blue vat dye having Color Index No. 1113, and 4 oz./ gal. of zinc hydrosulfite. One of the baths contained, in addition, 0.03 oz./ gal. of hydrated magnesium sulfate.

Titrations with standard indigo showed that, in the bath which did not contain the magnesium compound, the hydrosulfite consumption was about 50% more rapid than it was in the other bath in which the magnesium ions were present.

Example 20 (Mg/hs 0.002

A dye bath was prepared to contain 4 oz./gal. NaOH, 0.03 oz./gal. of a commercial dyeing paste of the blue vat dye having Color Index No. 1113, 1.0 oz./ gal. NaNOz, 4 oz./gal. sodium hydrosulfite, 0.003 oz./gal. triethanolamine, 0.003 oz./gal. hydrated magnesium sulfate, 0.003 oz./ gal. manganese chloride.

When this bath, along with a corresponding bath containing no magnesium, manganese, or triethanolamine was held at a temperature of 200 F. the hydrosulfite consumption was about three times as rapid in the unprotected bath as it was in that which contained the magnesium, manganese, and triethanolamine.

Example 21 (Mg/hs 0.071)

Example 22 (Mg/hs 0.036) The procedure. of thepreceding example was repeated In a second corresponding bath with no except 25 parts of MgSO4-7H2O were added to 100 parts of the real dye in the form of a dispersed press cake after which the mixture was dispersed to give a vat dye paste. When this paste wa used to make a 5% dyeing on cotton by the procedure of Example 6, a good strong blue shade was obtained. No additional MgSO4-7H2O was added.

Example 23 (Mg/hs 0.018)

The procedure of the preceding example was repeated except 12.5 parts of MgSO4-7H2O were added. A 5% dyeing of this dispersion on cotton caused excellent blue shades to be obtained.

Example 24 (Mg/hs 0.009)

The. procedure of the preceding example was repeated except only 6.25 parts of the MgSO4-7H2O were added. When this paste was used to make a 5% dyeing on lustrous viscose rayon, a good strong blue shade was obtained.

Example 25 (Mg/hs 0.004)

The procedure of the preceding example was repeated except 3 parts of the MgSO4-7H2O were added to' 100 parts of real dye in the form of a dispersed press cake. When cotton yarn was dyed in this bath by the procedure of Example 6, the hydrosulfite was decomposed less rapidly than when no magnesium salt was present in the e dye bath.

Example 26 (Mg/hs 0.009)

The procedure of Example 22 was repeated using magnesium lignin sulfonate (2.46% Mg by weight) and the hydrosulfite was decomposed less rapidly than when the magnesium salt was omitted.

Example 27 (Mg/hs 0.046)

The procedure of Example 23 was repeated except magnesium chloride was used. Cotton yarn dyed in this bath was colored a bright blue shade.

Example 28 (Mg/hs 0.009)

The procedure of Example 24 was repeated using, however, the dye of Example 1 of United States Patent No. 1,663,474 and a bright yellow shade was obtained.

Example 29 (Mg/hs 0.018)

The procedure of Example 23 was repeated using, however, the dye having Color Index No. 1106 and a good a strong bright blue shade was obtained.

Example 30 (Mg/hs 0.018)

The procedure of Example 23 was repeated using, however, the dye having Color Index No. 1112 and a good blue shade was obtained.

Example 31 (Mg/hs 0.018)

The procedure of the preceding example was repeated except the dye having Color Index No. 1101 was used and titration of the hydrosulfite in the dye bath showed that it was decomposed less rapidly than that which occurred when the dye bath contained no magnesium ions.

Example 32 (Mg/hs 0.018)

The procedure of the preceding example was repeated except the dye having Color Index No. 1098 was used. Satisfactory stabilization of the dye bath was obtained.

Example 33 (Mg/hs 0.031, 0.018)

frequently. Titrations with a standard solution of soluble. indigo showed that after 10 minutes the sodium hydrosulfite had all been consumed.

In a similar bath to which had been added the equivalent of 0.14 part of McS2O4, the hydrosulfite consumption was much less, there still being 4 parts of hydrosulfite present in the bath after 11 minutes.

In a third bath prepared exactly like the first, except that there were added to the bath 0.12 part of MgSO4-7H2O and 0.078 part of KMnO4. There were still approximately 3.5 parts of hydrosulfite remaining in the bath at the end of 12 minutes.

Example 34 (Mg/hs 0.003)

A, dye bath was prepared to contain 4 oz./gal. of NaOH and 2 oz./ gal. of hydroxylammonium sulfate. The bath was heated to approximately 200 F. and held at that temperature. 4 oz./ gal. of NazSzOa were then added and the bath stirred frequently. At the end of 20 minutes a titration with a standard solution of soluble indigo showed that less than 0.4 oZ./gal. of hydrosulfite remained in the bath. In a second bath to which had been added the equivalent of approximately 0.01 oz./gal. of MgSzOr more than twice as much hydrosulfite remained after the same period of heating.

Example 35 (Mg/hs 0.071)

parts of the dye of Example 21, in the form of a press-cake, were dispersed as in Example 21 and then colloidized by the general procedure of U. S. Patent No. 2,090,511, except 50 parts of MgSO4'7H2O were used to replace 50 parts of cerelose. The product was a colloidized vat dye containing a magnesium salt.

This powder was used to make a 5% dyeing by the procedure of Example 21 and the results were satisfactory.

Example 36 (Mg/hs 0.071)

The procedure of the preceding example was repeated except the magnesium salt was added to the defiocculated press cake before colloidizing. It was then blended with the requisite quantity of sugar.

Dyes having C. I. numbers 1106, 1112, 1098, 1101 and 1117 were colloidized by the methods outlined in Examples 35 and 36 and the colloidized powders were substituted for the paste in making dyeings according to the process described in Example 21. The results were satisfactory.

I claim:

1. A reducing bath suitable for high temperature vat dyeing comprising an aqueous solution of caustic alkali and a metal hydrosulfite, at least one stabilizer against over-reduction of vat dyes selected from the group consisting of inorganic nitrites, inorganic halogenates and nitrogen compounds in which a nitrogen atom isattached by at least one co-valent bond to oxygen and by atleast one co-valent bond to an element other than oxygenor nitrogen and at least .001 mol of ionized magnesium compound per mol of metal hydrosulfite the amountof said magnesium compound being less than the stoichio metrical equivalent relative to the metal hydrosulfite.

2. A bath according to claim 1 containing a vat dyestufi capable of over-reduction at elevated temperatures.

3. A bath according to claim 1 containing a pyranthrone vat dyestufi.

4. A bath according to claim 1 in which the stabilizer is alkali metal nitrite.

5. A bath according to claim 2 in which the stabilizer is alkali metal nitrite.

6. A bath according to claim 5 in which the vat dyestuff in a Vat dyestufi of the indanthrone series.

7. A bath according to claim 3 in which the stabilizer is alkali metal nitrite.

8. A bath according to claim 1 in which the stabilizer is a nitrogen compound linked by at least one co-valent bond to oxygen and by at least one co-valent bond to an atom other than oxygen or nitrogen.

9. A bath according to claim 8 in which the stabilizer is a hydroxylamine compound.

10. A bath according to claim 9 in which the bath contains a vat dyestuff capable of over-reduction at elevated temperatures.

11. A bath according to claim 10 in which the vat dyestufi" is a dyestufi of the indanthrone series.

12. A bath according to claim 1 in which the stabilizer is a nitrogen compound linked by at least one co-valent bond to oxygen and by at least one co-valent bond to an atom other than oxygen or nitrogen and the bath also contains a vat dyestufi capable of over-reduction at elevated temperatures.

13. A bath according to claim 1 in which the magnesium compound is magnesium sulfate.

14. A bath according to claim 1 in which the magnesium compound is magnesium ligninsulfonate.

15. A bath according to claim 1 in which the magnesium compound is associated with at least one other stabilizer selected from the group consisting of soluble manganese compounds and trialkanolamines.

16. A dyestuff composition comprising a vat dye sensitive to over-reduction and a sufiicient amount of an ionized magnesium compound so that when the vat dye is incorporated into a standard vat dye bath, using metal hydrosulfite as a reducing agent, there will be at least 0.001 mol ionized magnesium compound per mol of the metal hydrosulfite, the amount of said magnesium compound being less than the stoichiometrical equivalent relative to the metal hydrosulfite.

17. A composition according to claim 16 in Which the vat dyestutr" is a vat dyestutf of the indanthrone series.

No references cited. 

1. A REDUCING BATH SUITABLE FOR HIGH TEMPERATURE VATT DYEING COMPRISING AN AQUEOUS SOLUTION OF CAUSTIC ALKALI AND A METAL HYDROSULFITE, AT LEAST ONE STABILIZER AGAINST OVER-REDUCTION OF VAT DYES SELECTED FROM THE GROUP CONSISTING OF INORGANIC NITRITES, INORGANIC HALOGENATES AND NITROGEN COMPOUNDS IN WHIICH A NITROGEN ATOM IS ATTACHED BY AT LEAST ONE CO-VALENT BOND TO OXYGEN AND BY AT LEAST ONE CO-VALENT BOND TO AN ELEMENT OTHER THAN OXYGEN OR NITROGEN AND AT LEAST .001 MOL OF IONIZED MAGNESIUM COMPOUND PER MOL OF METAL HYDROSULFITE THE AMOUNT OF SAID MAGNESIUM COMPOUND BEING LESS THAN THE STOICHIOMETRICAL EQUIVALENT RELATIVE TO THE METAL HYDROSULFITE. 